The Gruithuisen Domes are a lunar geologic mystery. Based on early telescopic and spacecraft observations, these domes have long been suspected to be silicic in composition, perhaps somewhat similar to terrestrial volcanoes like Mt. St. Helens. Observations from LRO, particularly the Diviner instrument, confirmed that the Gruithuisen Domes are indeed much more silicic than most of the Moon, and they are distinct from the surrounding terrain, which has mostly been flooded by basaltic lavas. The basaltic lavas have less silica and are thus usually less viscous, so they flow and fill in topographic lows to create a flat plain. The silicic lavas are much more viscous, and when they erupt, don't flow outward easily, but instead form the domes that make up Gruithuisen.
While we are pretty confident that we understand this part of the story, the real mystery is how such silicic magmas could form on the Moon. On Earth silicic volcanoes are typically formed in the presence of two ingredients – water and plate tectonics. But without these key ingredients on the Moon, how did the Gruithuisen Domes form?
There are several theories about how silica-rich lunar magmas form. But in order to truly understand these puzzling features, we need to explore them on the surface, or return rock samples (or both!). And NASA is planning to do just that! A suite of scientific instruments will be delivered to this region, hosted on a Commercial Lunar Payload Services lander and rover in 2025. LROC images, like those featured here, will be critical for selecting a landing site that is both likely to provide the best opportunity to understand how these domes formed, and that will be safe for the lander and easy for the rover to traverse.
Hopefully in just a couple of years we will have answers to this longstanding lunar mystery!
Check out the Gruithesuin Domes below. Where would you land?
The Hortensius Domes are also a type of volcanic dome, but are compositionally different from the Gruithuisen Domes. They are made of basaltic lava, a dark and fine grained rock, similar to the surrounding terrain. They were likely formed by consecutive eruptions of lava from an underlying magma plumbing system. Read more here Hortensius Domes - Constellation Region of Interest
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This post was made in collaboration with Srinidhi Ravi at LROC SOC
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